Patent GB-1004519-A (GB)
Title: Hydraulic system with draft control for farm tractors
IPC: A01B63/111
Filed: 1962-05-11
Abstract: 1,004,519. Tractor power-lift mechanisms. INTERNATIONAL HARVESTER CO. May 11, 1962 [May 13, 1961], No. 18184/62. Heading A1P. A tractor hydraulic control system incorporating "position control" and "automatic draft control" is characterized by a piston valve 21 which is acted on on one side by the pressure of the main oil flow from pump 15 and on the other side by oil passing through a throttle bore 25. For draft control, movements of the upper link 3 of the hitch linkage operate a lever 58 to move a rod 10 whereby a lever 11 is moved to operate the main control valve 28 which controls the supply of pressure fluid to the working ram 5. A hand lever 12 sets the required amount of draft by operating lever 11. For position control a hand lever 72 operates the main control valve 28 through a lever 44 which is connected to the rock shaft 6 by a rod 66 by a pin and slot connection 68 which renders position control ineffective when draft control is used. In the neutral position of the main control valve as shown in the Figure the main supply and return lines from the pump to the cylinder i.e. via recesses 31 and 30 are cut off and pressure oil flowing through the throttle bore 25 of the piston valve 21 is returned to the reservoir 16 via lines 27 and 32 so that a pressure build up on the right end of the valve 21 moves it to the left as shown to open a return line 24 to the reservoir for the main oil flow. When the main control valve is moved to the right (the slow-lift position) return line 32 is cut off from supply line 27 so that oil is supplied through throttle bore 25, lines 27 and 35 to the ram. In this case pressure builds up on the left hand side of valve 21 to overcome the ram pressure and the valve 21 moves to partially close the return line 24 to provide equal pressure on both sides of the valve. When the main control valve is moved still further to the right (the quick-lift position) pressure is applied to the ram through lines 34 and 38 and this pressure holds check valve 36 in place to close line 35 until oil passing through throttle bore 25 builds up to the full supply pressure and valve 21 completely closes the return 24 under the action of spring 26. When the main control valve is moved to the left of the position shown, i.e. the lowering position, the annular recess 30 is connected to the return line 33 to the reservoir, thus releasing pressure in the ram. The rate of movement of the ram in this case is controlled by a throttle 48 operated by a hand wheel 55. This throttle has a bypass 58 to operate when pressure is supplied to the ram. The hand wheel 55 also has parts 50 and 56 which can be moved to isolate the ram 5 and connect the circuit to an auxiliary ram 54. To avoid the possibility of the main control valve having a throttling action between the supply and return lines 27 and 32, the valve is operated by levers 11 and 44 through a piston 77 having a pin and slot connection 78, 79 with the valve and having at one end a space 81 which fills with the pressure fluid if throttling occurs and is sufficient to move the valve to the right relative to the piston against the action of spring 42 thus clearly cutting off the line 32 from line 27.